Find out how the action unfolded below.
Good evening everyone! The League Cup takes centre stage this week and there are plenty of reasons to expect some memorable upsets. League One outfit Sheffield United will be hoping to book a semi-final spot, as they welcome a faltering Southampton to Bramall Lane.
It was all going so well for the Saints until a couple of weeks ago. They were second in the Premier League and being talked about as Champions League contenders. However, a run of four defeats has cast doubt over their European aspirations and has seen them drop down to fifth in the table.
Sheffield United will no doubt see this evening's game as the perfect time to face their top flight opponents, but they aren't exactly in the greatest run of form themselves. They have drawn three and lost one of their past four league matches and have fallen well off the pace for an automatic promotion spot. Right, before I go any further, let's take a look at the teams for this evening.
SHEFFIELD UNITED XI: Howard, Harris, Basham, Flynn, Doyle, Murphy, McNulty, McEvely, Campbell-Ryce, Reed, Cuvelier
SOUTHAMPTON XI: Forster, Clyne, Fonte, Gardos, Targett, Wanyama, Schneiderlin, Ward-Prowse, Bertrand, Mane, Long
SHEFFIELD UNITED SUBS: Baxter, McGahey, Higdon, Turner, Kennedy, Adams, Wallace
SOUTHAMPTON SUBS: K. Davis, Yoshida, Alderweireld, Mayuka, Isgrove, Reed
TEAM NEWS: So, those are our teams for this evening. Each manager has made five changes to the starting XI from their league games at the weekend. It is still a strong lineup from the Saints, who welcome back Morgan Schneiderlin from injury. The visitors have also been forced into a change up front, as Shane Long comes in for the suspended Graziano Pelle.
Just a month ago Southampton were being hailed as one of the standout teams of the season, with manager Ronald Koeman and his players winning praise for their fine start. Following a 2-0 victory over Leicester City in early November they were in second place and trailed leaders Chelsea by four points.
That match was followed by an international break and the Saints just haven't been the same since. On their return to league action, they salvaged a late draw against Aston Villa, before falling to a 3-0 defeat against 10-man Manchester City, which signalled their first loss in nearly two months.
A few days later they travelled to the Emirates to face Arsenal and slipped to a 1-0 defeat thanks to Alexi Sanchez's late goal. They then fell to their second home defeat of the season after losing 2-1 against Manchester United, in a game that saw the visitor register just three shots on goal.
It was their first tough run of the season against three of the league's best sides and many expected them to bounce back with a win at the weekend against lowly Burnley. That didn't prove to be the case, as Ashley Barnes struck a second-half winner to give the Clarets a priceless three points in their battle against relegation.
Such a poor run has raised question marks over their top-four challenge, but they are only two points behind West Ham United in the final Champions League qualifying spot. A win tonight would certainly boost their confidence, ahead of clash against Everton on Saturday. Their run over the festive period certainly looks tough, with three-consecutive league games against Chelsea, Arsenal and Manchester United to come.
This will be the second time that Southampton have played lower league opposition in this cup run. They faced Millwall when they entered in the second round and claimed a 2-0 victory. It was much tougher in the third round, as they visited Arsenal away from home, but a stunner from Nathaniel Clyne helped them to a 2-1 win. Last time out against Stoke City there was plenty of drama as they threw away a two-goal lead, before Graziano Pelle scored a late winner to earn them a 3-2 victory
Well, the hosts would have been looking at Southampton's recent form believing that they could pull of an upset, but it hasn't exactly been easy for them either. The Blades are down to fifth in the League one table after a run that has seen them win just one from their past six league games.
Aside from two FA Cup victories over Crewe Alexandra and Plymouth, their last win at home in the league came nearly two months ago against Yeovil Town. The club has also been embroiled in plenty of controversy over the past month after allowing Ched Evans to train with them following his release from prison, before retracting the offer due to a public backlash.
This isn't the first time in this season's League Cup run that the Blades have played against Premier League opposition. In the second round they faced West Ham United at Upton Park and took the Hammers all the way before triumphing in a penalty shootout. They also enjoyed a great run in the FA Cup last season, beating Aston Villa and Fulham on their way to the semi-final.
2 - Sheffield United have scored two and conceded 15 in their last five League Cup ties against sides from a higher division. Blunted. — OptaJoe (@OptaJoe) December 16, 2014
Tonight's match will be the first game between the sides in over five years and their first ever meeting in the League Cup. It was Sheffield United who won on that occasion, by claiming a 2-1 win at St Mary's Stadium. The Blades actually have the better record head to head record, by winning 24 and losing just 16 of their 53 encounters.
PREDICTION: I'm sure that many people will be thinking that this game will provide some shocks, but I have to go for the visitors. They may have hit a bad patch, but they should have far too much for Sheffield United side who have struggled recently in front of goal. So, for that reason I'm going to tip the Saints to claim a 2-0 win and book their place in the semi-finals.
As we edge closer towards kick off, let me remind you that we have live coverage of all of this week's League Cup ties, including Chelsea's clash against Derby County right here.
The players have just made their way out onto the pitch to a rapturous reception from the home crowd and we are now just a few minutes away from the start of the match.
KICKOFF! We are underway at Bramall Lane!
OFF THE BAR! It's almost a disastrous start for Southampton as the visitors hit the bar. A free kick from out wide is whipped into the box and Forster comes to claim. The keeper looks as if he has the ball, but he spills it into a crowded box. It falls to McEveley who hooks the ball goalwards, but it ends up bouncing off the bar and going out.
The Saints are seeing plenty of the ball as expected, but they are yet to get in behind and test the keeper. The Blades have put them under a good deal of pressure and are closing their opponents down quickly.
It's very much all Southampton at the moment and the hosts just can't get a foot on the ball. They surely can't play like this for the whole match, as a team like Southampton will bide their time and gradually work a way through their defence.
It has been a scrappy opening period and aside from hitting the bar early on, chances have been few and far between. Southampton have had a dangerous chance from a set piece out wide, but the delivery from Ward-Prowse failed to beat the first man.
Nigel Clough will be pleased at the way his side have started tonight's match and they have begun to see the ball in more advanced areas of the pitch. They are certainly giving Southampton something to think about and they are looking particularly good out wide.
It's safe to say that this game has yet to really spark into life and we have only seen one effort on goal so far from each side. It may be early, but it does seem that this game could be going all the way to a shootout.
The hosts win themselves a free kick in a very promising position around 25 yards out from goal when Fonte hacks down Cuvelier. The Belgian steps up to take the resulting set piece, but he ends up curling the ball a few inches too high of the bar.
Sheffield United have really grown into this game and they are starting to pass it around nicely. They are certainly showing no signs of fear against their Premier League opponents and if Cuvelier's touch on the edge of the box following a great team move was better, then they could have been in behind.
It's not been a great game for the two keepers, as Howard almost costs his side a goal. He comes to collect a Ward-Prowse free kick, but just as Forster did earlier, he drops the ball into the danger zone. Mane is the closest Saints player there, but he can't get a touch and the defence manage to scramble the ball clear.
YELLOW CARD! Clyne brings down Harris just outside the box with a crunching challenge and picks up the first booking of the evening.
YELLOW CARD! The Saints are handed a second booking in the space of a minute when Mane goes in with a wild tackle on Murphy.
SHOT! This would have been some way to score your first goal for Sheffield United. The Blades come forward, with 17-year-old Reed picking up the ball in the midfield. He charges towards the box and fizzes in a powerful drive, but it goes just over the bar.
CHANCE! The Blades are well on top now and they work a fantastic opening here. Basham creates the chance, as he goes on a great run down the right, before standing up a decent cross. The visitors are unable to get it clear and the ball lands at the feet of Cuvelier, who then slices a shot wide of the far post.
Mane has already been booked and he really should be sent off here. A long free kick is punched away by the Howard, but Mane comes crashing into the keeper. He was very late and Chris Foy had a word with him, but decided against showing him another card.
YELLOW CARD! The Blades have now picked up their first booking, with Harris shown yellow for hauling down Long, as the striker looked to race through the midfield.
There is no love lost between these sides and at the moment tensions are running high after each challenge. With half time fast approaching, the two managers will have to tell their players to clam down, as a red card looks very likely at the moment.
There are going to be two minutes of stoppage time at the end of the first half.
HALF-TIME: Sheffield United 0-0 Southampton
The whistle blows for half time at Bramall Lane and it is still goalless between Sheffield United and Southampton. Although we have not seen many chances on goal, the game has threatened to boil over and we are set up for an intriguing second half.
It's no surprise that Southampton have had more of the ball, with 59% of the possession in the opening 45 minutes. They have only had three attempts on goal, which is one less than United's four, but neither side have been able to hit the target so far.
We're just a few moments away from the start of the second half and both teams are going to make a change. For the hosts, Adams is coming on for Campbell-Ryce, while the saints have brought on Isgrove for Targett.
We are back underway at Bramall Lane.
PENALTY APPEAL! The visitors feel that they should be given a spot kick when Gardos sees his header blocked by the arm of McEveley inside the box. The replays confirm that the ball hit the defender, but it would have been a harsh call for the referee to give anything there.
CHANCE! What a huge moment this is for Sheffield United. Harris does brilliantly down the left and whips a great ball across the box that picks out Murphy unmarked at the back post. He has to take the shot early as the defence are closing in on him quickly, but he can only blaze his effort wide of the post.
CHANCE! The game is starting to open up now and both teams have come close to a goal in this half. This time it's Southampton who threaten, as Long flicks on cross into the path of Ward-Prowse who directs a header inches wide of the post.
CHANCE! This is as close as either side have come to a goal. Murphy whips in a dangerous cross from the left, that is heading towards the bottom corner until Forster turns the ball away. The rebound then drops to Adams, but his fierce attempt is well blocked by Clyne.
YELLOW CARD! Cuvelier is booked for the hosts after clattering into Schneiderlin.
CLOSE! Ward-Prowse has looked the man most likely to score for Southampton and he has a good chance from a free kick in central position just outside the box. He looks to bend a strike into the top corner, but his effort flies narrowly over the top.
YELLOW CARD! The cards are piling up now and Basham goes into the book for bringing down Mane by the left-hand touchline.
SAVE! The Blades are coming closer and they threaten once again when Adams tees the ball up for Doyle to fire in a drive from the edge of the box. He could not have struck it much better, but it was at a good height for Forster to tip the ball over the bar.
GOAL! Sheffield United 1-0 Southampton (Marc McNulty)
We've got a shock on our hands now as Sheffield United take the lead. It's a scrappy goal, as Bob Harris fires in a low free kick from the edge of the box that Forster can't hold and the ball squirms along the line until McNulty dives in to force the ball home. It may have already crossed the line before McNulty got the touch, but the hosts won't care one bit.
Southampton can't be too surprised by how things are going at the moment, as the warning signs were there. The Blades have created a number of decent chances since the break and now their main aim will be holding onto this lead, with a place in the last four at stake.
YELLOW CARD! Wanyama becomes the sixth player of the evening to be shown a yellow card after a foul on Flynn. That is bad news for him, as he will miss his side's game against Everton at the weekend.
SAVE! The Saints are beginning to pile on the pressure and Mane finds some space on the edge of the box, before drilling in a low strike that Howard manages to divert around the post with his boot.
SUBSTITUTION: That was Mane's last piece of action, as he heads off to be replaced by Mayuka.
CHANCE! McNulty has already scored one and he really should have another here. He is slipped clean through on goal by Murphy's fine pass, but he is unable to slot his low strike beyond the big frame of Forster.
SUBSTITUTION: It's a second change for the Blades, as Baxter replaces Cuvelier.
SUBSTITUTION: Southampton make a change too, as Alderweireld comes on for Wanyama.
Southampton continue to attack and they almost create a chance when Long knocks a header across the box, but none of his teammates are able to reach the ball in time. As they are pushing so many men forward now, there is plenty of space for Sheffield United to exploit down the flanks and you would not be surprised to see them grab a second on the counter.
SUBSTITUTION: The hosts make their third and final change of the evening as Reed comes off to be replaced by Wallace.
YELLOW CARD! Schneiderlin is the latest in a long list of players to go into the book this evening for a foul on Wallace. It means that like Wanyama before him, he will miss Southampton's next match.
Less than a minute after being booked, Schneiderlin is lucky to stay on the pitch. He gives away another foul and then kicks the ball away right in front of the referee. The hosts complain that he should be shown another card, but he is spared by Foy.
SAVE! The hosts have had a couple of chances to seal the win now and they come close again when Murphy is slipped through, before drilling in a strike from the edge of the box that is beaten away by Forster. They will be kicking themselves if they allow this lead to slip in the final minutes.
RED CARD TO SOUTHAMPTON! I said that there was a strong chance we would see a red card and it has come in the final minute. It's Gardos who has to go, as he hauls down McNulty just outside the box with the United player heading through on goal.
YELLOW CARD! Southampton have lost their disciple tonight and Fonte is shown a card for a poor tackle from behind on McNulty. It really has been a night to forget for Koeman's men.
FULL-TIME: Sheffield United 1-0 Southampton
It's all over at Bramall Lane and Sheffield United have pulled off a memorable cup upset. They are into the last four thanks to Marc McNulty's goal. Southampton have now lost their last five games and it couldn't really have gone much worse for them today.
That's all we've got time for here after a memorable night for the Blades. We've got more live coverage of this week's League Cup action for you tomorrow, but until then, I hope you enjoy the rest of your evening. Goodbye!